Depends on the batteries themselves, and how they've been treated.
For a battery
study I did last year, I got hold of 4 golf-cart sized gel batteries which were 10 years old and had been used aboard a sailboat for that time. The sailboat stayed at the dock
during the week, and only did weekend jaunts. The batteries were left on charge most of the time.
All four tested pretty good. One of the four tested almost new, and is still testing just great. (Testing done with a pretty expensive piece of gear
which measures internal resistance; also, of course, with digital voltmeter, and 20-hour load testing while plotting voltage decay).
So, with luck, I'd say gels can last a pretty long time on a boat if they're taken care of!